What is Light: Definition and 1000 Discussions

Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths). This wavelength means a frequency range of roughly 430–750 terahertz (THz).

The primary properties of visible light are intensity, propagation-direction, frequency or wavelength spectrum and polarization. Its speed in a vacuum, 299 792 458 metres a second (m/s), is one of the fundamental constants of nature, as with all types of electromagnetic radiation (EMR), light is found in experimental conditions to always move at this speed in a vacuum.In physics, the term 'light' sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. Like all types of electromagnetic radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed. The absorbed energy of the electromagnetic waves is called a photon and represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave instantly collapses to a single location and this location is where the photon "arrives". This is what is called the wave function collapse. This dual wave-like and particle-like nature of light is known as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.
The main source of light on Earth is the Sun. Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has effectively replaced firelight.

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  1. G

    B Can an observer perceive he is traveling faster than light?

    I have seen thought problems with an observer on a train or in a station, etc., but I have not seen ones with the observer traveling at relativistic speeds. It seems to me that at sufficient speed he would observe himself exceeding the speed of light due to the slowing of time. This seems like...
  2. MeAndMyLucidLife

    Photo-current and Intensity of light

    Homework Statement If the frequency and intensity of a light source are both doubled, show that the saturation photo-current remains almost the same. 2. Relevant Graph https://cnx.org/resources/e73bf218926bd39de4a52bafe3a82d04bbf73542/CNX_UPhysics_39_02_photoexp1.jpg The Attempt at a Solution...
  3. Jett

    Understanding a Generalization of Snell's Law

    I have been working a project of creating a simulation of thin-film interference in a 3D modeling program, Blender, to be able to create materials that would use its effects. While uncommon, I would love for it to be able to correctly calculate reflectance and transmittance when the incident...
  4. N

    Optical Smoothest yet not specular metal or plastic sheets?

    I'll get into the details why I need this for below but first I'll explain what I'm looking for. I basically need a flat sheet of aluminum or any plastic in grey or black color that doesn't provide specular reflections, yet is smooth enough to not have any noticeable texture when looking with...
  5. vincent

    I Does a Photon Carry Velocity of Its Source?

    Earth is moving through space. If I throw a ball straight up, it falls down to me and is not left behind. This is obviously because it already carries the momentum of Earth's movement. But if I fire a photon straight up, does it also carry a velocity component in direction of Earth's movement...
  6. Antonio dela Vinci

    B What if light were influenced by gravity?

    How would the world be if light didn't travel in a straight line? I was wondering how it would affect life.
  7. P J Strydom

    B Measuring Michelson–Morley light beams

    I was away for a few months and thought I might pose this thought I am working on, on this forum when I return. There was a very help full person who assisted me in the finer explanation of the Lorenz transformation a few months ago, and I really learned a lot about SR and GR. A few weeks ago, I...
  8. Bishal Banjara

    How to reduce the integral equation for light deflection?

    1. At pg.212, Hartle book (2003) writes equation 9.81 as an approximation of 9.80, directly. 2. $$ΔΦ=\int_0^{w_1}\frac{(1+\frac{M}{b}w)}{(1+\frac{2M}{b}w-w^2)^\frac{1}{2}}dw$$ equation(9.80) $$ΔΦ≈\pi+4M/b$$...
  9. J

    Do Blue Light Filters Actually Work?

    I have both my iPhone and desktop computer set to a "night light" mode, and obviously my eyes appreciate the warmer colors at night. But what I'm wondering is this: Do they actually work? Here's what got me thinking: I have a pair of Elvex glasses rated to block all blue wavelengths, and I...
  10. alejandromeira

    B Time & Limit Velocity (Speed of Light)

    Hello. Today I've thinking about limit velocity and speed of ligth. We know that material particles can't achieve that speed, also when the speed of particles increases your own clock walks slowly. In the particular case of ligth your speed don't move anything. This it a explanation of why...
  11. D

    Light Speed: Distance & Time Explained

    Hello forums, I'm kind of new nice to meet you guys and girls, Could someone please explain this to me? D=S * TTherefore Distance = Speed of light * TimeAnd then Distance = Speed of light + 7 * TimeWhat is Distance and what is time?
  12. Killtech

    I Constancy of the speed of light

    I am trying to get a better understanding about the constancy of the speed of light which is a well-established axiom of current day physics. for the start i want to understand how it is experimentally established and how these results are interpreted. My difficulty here is that this seems to be...
  13. Hans Nelsen

    B How Does Age and Distance Affect Light from Distant Galaxies?

    Since more distant objects are older, and the farthest objects we receive light from are billions of years old, or rather, their light is billions of years old, in what way, or how different would the distant universe be, if we could see the light it generates today? For that matter does light...
  14. A

    Viewing a flame in bright light?

    im trying to figure out if there's a way i can see the FULL flame from my torch (MAP) without having to be in an potentially unsafe dark environment as yall may know MAP gas burns a very light blue<teal colour that's very hard to see in normal light levels using the interwebs, all I am finding...
  15. K

    Min Max problem: the shortest distance for a light ray

    Homework Statement Homework Equations Minimum/Maximum occurs when the first derivative=0 GM≤AM: ##~\sqrt{xy}\leq\frac{x+y}{2}## The Attempt at a Solution [/B] If the sum of squares of the distances (setup 2) in an arbitrary point is bigger than the sum of the squares of the shortest...
  16. J

    I Why don't red dwarf stars emit white light like the Sun?

    Blackbody radiation spectra of hot light sources ( stars in this case) should have the full visible light spectrum and still stars less massive than the Sun emit reddish light while stars more massive than the Sun emit blue light. How is the blackbody radiation spectrum altered by the star mass?
  17. Pushoam

    I Light : Wave - particle duality

    Interference pattern made by light shows the wave nature of light and photoelectric effect shows particle nature of light. So, what is light? According to the photoelectric effect, light consists of photons with energy E and momentum ## \vec p##. According to the interference pattern, we...
  18. P

    Classically communicate information faster then light?

    Where in this though-experiment do I get it wrong? Even though no mass can travel faster then c, maybe information can? And I'm not talking about quantum entanglement etc. Consider a pipe, filled with balls that are very tightly arranged. If I push the outermost ball on one side of the pipe...
  19. Luis Babboni

    I Understanding the Speed of Light in Ether: A Historical Perspective

    Hi people! May be this could be an historical question. Before Einstein, it is suppoused that speed of light in ether is always the same, I´m right? I mean, if observer O is at rest respect ether and observer O´ is moving respect ether and O´ send a photon to O, the speed at which that photon...
  20. Korak Biswas

    A Quantum Optics: Squeezed light generation

    Theoretically, squeezed state can be generated using a process called 'parametric down conversion'. I was going through the literature and found that people performed 'second harmonic generation' before doing 'parametric down conversion'. Can anyone tell me why simple harmonic generation is...
  21. M

    Light wavelength transmission through ice

    I am studying hailstones. I have set up an experiment to pass colored light through hail, one color at a time for the colors of the visible spectrum, Red, Orange, Yellow, Green, Blue, Indigo and Violet (ROYGBIV). I have measured the dominate wavelength for each color with a photo spectrometer...
  22. AGuglielmone

    B Are all wavelengths of light possible?

    I just had the thought that atoms emit light at quantized levels but that would seem to imply that only certain energy levels could possibly exist instead of a complete spectrum. But, if light is traveling down or away from a gravitational field the frequency gets shifted. Would this make it...
  23. S

    Does Light Travel Through Fiber Optic Cable Generate EMF?

    Does light traveling through a fiber optic cable generate any sort of detectable electromagnetic field? Please forgive the stupid question. It’s something that popped into mind recently and google hasn’t adequately answered for me. I’m not a scientist or physicist. :blushing:
  24. Shivam

    Does the Photoelectric Effect Prove the Particle Nature of Light?

    It is said that photoelectric effect of light proves that light has particle nature, but which property is shown by light in the photoelectric effect which is also the property show by things having particle nature? Even in the photoelectric effect the energy is transferred in the form of wave...
  25. P

    Measuring small differences in the phase of light

    Recently I read something in NASA Tech Briefs about an instrument that could measure phase differences as small as .001 degree in light. I would like to create an instrument that could do this for every pixel in a picture, even if it was only a few hundred pixels (such as a 20x30 pixel array)...
  26. KezPhysics

    How long does it take a 1ms pulse of light to travel 20m

    Homework Statement If you create a 1 ms light pulse by turning a flashlight on and off, how long will the pulse take to reach the other side of the room 20m away? (in air at standard T and P) what is the length, in m, of the pulse? I realize that light has a wavelength and when you pulse it...
  27. jedishrfu

    B Imaging of Light as a Particle and a Wave

    In this article, researchers have used electrons to image light as both particle and wave at the same time. https://phys.org/news/2015-03-particle.html
  28. J

    I Traveling at Light Speed: Is a Photon Stationary?

    If you were to travel alongside a train, as fast the train, to you the train would seem stationary. I read that if you were to travel along a photon of light, as fast as the speed of light, that photon would not seem stationary. Is this true? If so, why?
  29. P

    Why is a state with large number of photons not classical?

    In the last paragraph of these notes, https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/lecture-notes/MIT8_04S16_LecNotes3.pdf, it says how a state with large number of photons is not classical. Why is that? I thought quantum mechanics' laws were most applicable when we...
  30. D

    I Explanation for the hyperbolic array of light rays.

    Hello, Recently, a solar power tower plant was founded next to where I work. Since it's the tallest object in the area, it's quite hard to miss it. But apart from that, every morning the reflected light is arranged in a hyperbolic- like way, as you can see in the picture. Does anyone have a...
  31. Erickly

    Film Thickness for Minimum Reflection of Monochromatic Light: How to Calculate?

    Homework Statement Monochromatic light of wavelength, λ is traveling in air. The light then strikes a thin film having an index of refraction n1 that is coating a material having an index of refraction n2. If n2 is larger than n1, what minimum film thickness will result in minimum reflection of...
  32. Erickly

    Rayleigh's Criterion + Moon (What am I doing wrong?)

    Homework Statement Suppose you wanted to be able to see astronauts on the moon. What is the smallest diameter of the objective lens required to resolve a 0.60 m object on the moon? Assme the wavelength of the light is near the middle of the visible spectrum: 550 nm yellow light. (in m) A...
  33. S

    Reflection of only visible (safe) light on moon/mars

    I have seen many articles lately regarding planned manned missions to the moon and Mars but the question of radiation protection constantly comes up. Engineers keep proposing various shelter designs that use local materials (regolith) as a concrete base to absorb harmful radiation so that the...
  34. T

    I Why does the expanding universe redshift light?

    How does the expansion of space cause light to lose energy?
  35. R

    B Speed of Light Slowed for SuperBig Observer?

    How fast would the light travel for a SuperBig observer... immagine his head is the size of the Sun, and he is sitting in a room, then he decides to turn on the light in the room... if he is so big, would he have to wait some seconds until the light reaches his eyes? Would it mean that for this...
  36. R

    How do electromagnetic waves from the sun vary in wavelength?

    How does the sun emit or radiate its electromagnetic waves? https://imgur.com/odqclja Is it like the first picture where it might be spaces in between the waves or more like the picture under it where the waves are uni formally (I think is the right word) with no space in between them? Or...
  37. B

    Questions about neutrinos compared to the nature of light

    First question: Within experimental error all measurements of neutrino and light speed in vacuum are consistent with c, but one way speed measurement of light is well proscribed, so is a one way speed measurement of neutrinos also proscribed? I have more questions, but maybe just clarify this...
  38. cliffhanley203

    Wave / particle duality of light

    Homework Statement I've just read that light can behave like a wave at times, and a particle at other times. How does the light from the Sun traveling towards Earth behave? A wave? A particle? Or both? And is it in any sense something that actually starts at the Sun, travels across space and...
  39. R

    If light speed (not in a vacuum) was about human walking speed ....

    If light speed (not in a vacuum) was about human walking speed ... Did any sci fi writer try that?
  40. C

    I Measuring 1-Way Speed of Light: 15m Fibers & 1GHz Pulse Generator

    Drive two 15m long optical fibers extended in opposite directions with a 1GHz pulse generator. The recievers at the end of each fiber are now syncronised sources of 1GHz pulses. Connect them to pulse counters, one of which provides a signal whenever a pulse train is present. Connect this...
  41. Sjm_dynamo

    B Why is the speed of light a constant?

    Why is the speed of light a constant?
  42. stoky

    Do car windows and windshields block ultraviolet light?

    I'm curious if car windshields / windows block UV (ultraviolet) light, the one from either the sun, or UV flashlights, like the Convoy S2+ Nichia 365nm.
  43. N

    Polarization of light using Mueller matrix

    Hi everyone First of all, I am a computer science student and I have a question regarding the polarization of light as stated in an article entitled "Multi-stage quantum secure communication using polarization hopping" by Rifai et al.,2015. Given the Mueller matrix: The input of light state is...
  44. H

    Can we use the momentum of light to propagate spacecrafts in space?

    as we know light has momentum so theoretically we can use it but is it practical? (also this is it that light only exerts force if incident on something?)
  45. Sandeep T S

    Uncertainty and the speed of light?

    By Uncertainty Principle speed of a particle cannot be constant. then speed of a single photon not?
  46. N

    On board a laser powered solar sail

    Can an on board laser be used to propel a solar sail spacecraft if the laser is pointed at the sails ? Would Newtons third law affect the laser and maybe prevent the ship from moving ? Thank you for answering my very ignorant questions. NineNinjas911
  47. H

    B What would happen if the speed of light were different?

    if speed of light were not 3*10 ^8 m/s and something else would it affect the reality ?
  48. P

    Assumptions made when deriving the speed of light

    Homework Statement The problem I have been working with recently has been deriving the speed of light using maxwells equations, however in order to do this I must make two assumptions; there is no net charge or displacement currents in the space in which I am attemptin to derive the speed of...
  49. BadgerBadger92

    How is light related to charged particles?

    I did a little more research, please clarify. So the electric field is directed to the proton and that emits light? Can you get into more detail? I am not educated in math so this is hard to understand at this point. I understand how electrons emit light but not protons. So is the magnetic...
  50. Timothy Schablin

    Have you heard of the mirage effect on the sea?

    I remember reading about how a city could be seen across a sea at certain times. But normally would not be seen because of Earth's curvature. At certain times however, it could be seen because of light bending. I can't remember what the cause was though. Anyone know about this and what caused it?
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